A Million Miles to Earth: USC Instruments Find a Place in the Sun

by Eric Mankin, courtesy of USC News Service

A USC-designed and -built instrument installed in a solar observatory
space vehicle is giving scientists long-sought, accurate values for a
key component in the Earth's energy diet. The Solar and Heliospheric
Observatory (SOHO), launched Dec. 2, 1995, contains a sophisticated
package designed to measure the flux of extreme ultraviolet light (EUV)
and X-rays produced by the sun, as well as energetic particles.

A schematic of SOHO's "halo orbit."
The orbit is designed to allow SOHO to observe the sun
continuously, without ever passing into the Earth's shadow.

SOHO's unique, stable observation position -- 1 million miles from
Earth-sun line at a "Lagrange point" where the pulls of the Earth and
sun balance -- allows uninterrupted solar observations. "We have
observed a 27-day modulation of the solar flux associated with the
rotation period of the sun," said solar astronomer
Darrell L. Judge, under whose direction one of the
key solar EUV-monitoring instruments on the spacecraft, the CELIAS/SEM,
was built. "Such variability is only observable through continuous
monitoring of the sun, such as SOHO provides." The instruments's name
is an acronym for Charge, Element and Isotope Analysis System/ Solar
Extreme-ultraviolet Monitor.

The EUV spectrum is blocked by the Earth's atmosphere, making
ground-based measurements impossible. Scientists have previously
measured the flux of this radiation either by suborbital rockets rising
briefly above the atmosphere, or by satellites that pass behind the
Earth and thus out of view of the sun for part of their orbits. "SOHO
permits the accumulation of an around-the-clock database over an
extended period of time," impossible with the earlier platforms, Judge
noted.

The CELIAS/SEM, first turned on Dec. 16, was briefly turned off for
operational reasons Dec. 20, and has operated continuously since then,
providing extremely high-quality data. The observations taken from Dec.
16, 1995, to now have all been obtained during solar minimum, the time
when the sun is at its lowest level of activity, and have found
larger-than-expected variations in the amount of ultraviolet light
produced by the sun.

SOHO as it would look to an astronaut
visiting it in its "halo orbit" above the Earth, with its flat solar
panels deployed.

During this period the CELIAS/SEM observed a flare episode in which the
soft X-ray flux increased by a factor of 100. "We look forward to
observations of the sun during a more active period." Judge said. The
solar EUV data is important in planetary science studies because
earlier studies have shown that EUV absorption in the upper atmosphere
"produces heating, ionization, and excitation of atomic and molecular
species, thus leading to complex chemical and transport processes in
the affected atmospheric regions," according to a report on the
experiment presented in June, 1996, at a solar workshop. A complete
paper on the results will appear soon in the Journal of Solar Physics.

Scientists have attempted to model both planetary and solar atmosphere
processes, but definitive tests of the models depend on more accurate
measurements of the EUV flux than have so far been available. The SOHO
project is being carried out by the European Space Agency (ESA) and the
U.S. National Aeronautics and Space Administration (NASA) as a
cooperative effort.

Other institutions involved in the CELIAS/SEM observations on board the
SOHO spacecraft include the Max Planck Institute for Extraterrestrial
Physics, Max Planck Institute for Aeronomy, the Braunschweig Technical
University and the Physikalish-Technishe Bundesanstalt, all of Germany;
the University of Bern, Switzerland; University of New Hampshire;
University of Maryland; University of Arizona; JPL; the Institute for
Space Physics in Moscow; and the Physics Laboratory of the National
Institute of Standards and Technology in Maryland.